Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual Men's u20 tem um Men's 45 de n/A?
 ###Tabelas: CREATE TABLE top_5_winners (top_5_winners_id AUTO_INCREMENT PRIMARY KEY, Year INT, Putri Pariwisata Indonesia VARCHAR, 1st Runner Up VARCHAR, 2nd Runner Up VARCHAR, 3rd Runner Up VARCHAR, 4th Runner Up VARCHAR) CREATE TABLE dragon_laws_television_films (dragon_laws_television_films_id AUTO_INCREMENT PRIMARY KEY, Year INT, Series title VARCHAR, Chinese title VARCHAR, Role VARCHAR, Channel VARCHAR) CREATE TABLE honours (honours_id AUTO_INCREMENT PRIMARY KEY, Year VARCHAR, Division VARCHAR, Place VARCHAR, Played INT, Drawn VARCHAR, Lost VARCHAR, G.F. VARCHAR, G.A. VARCHAR, G.D. VARCHAR, Points INT) CREATE TABLE results_by_year (results_by_year_id AUTO_INCREMENT PRIMARY KEY, Year INT, Men's Open VARCHAR, Women's Open VARCHAR, Mixed Open VARCHAR, Men's u20 VARCHAR, Women's u20 VARCHAR, Senior Mixed VARCHAR, Men's 30 VARCHAR, Women's 30 VARCHAR, Men's 35 VARCHAR, Women's 35 VARCHAR, Men's 40 VARCHAR, Women's 40 VARCHAR, Men's 45 VARCHAR, Men's 50 VARCHAR) CREATE TABLE background (background_id AUTO_INCREMENT PRIMARY KEY, Model VARCHAR, A300 VARCHAR, A310 VARCHAR, A320 VARCHAR, A330 VARCHAR, A340 VARCHAR)  
SELECT Men's u20 FROM results_by_year WHERE Men's 45 = n/a